The former information commissioner told the Leveson inquiry that his office had been right not to prosecute newspapers despite overwhelming evidence they were "driving the trade" in information obtained illegally by private investigators.

Richard Thomas told the inquiry: "I was of the view, 'Thank goodness we had not prosecuted journalists' because of some of the problems we had encountered."

During evidence that lasted for an entire day, he said taking legal action against the print media "would have bogged down the office for many years". Thomas added that he agreed with the assessment of a third party, who told him in an email: "They would have gone all the way to Strasbourg and challenged the ICO every step of the way."

Thomas had earlier denied that he had ordered his office not to pursue newspapers because he was wary of the power of the press.

Alec Owens, the investigator at the Information Commissioner's Office who led a 2003 probe into private investigators, told the inquiry last week he had been told by ICO executives newspapers were too big to target.

The former police officer claimed Thomas and his deputy at the time, Francis Aldhouse, were "frightened" of the media. Aldhouse denied this in his own evidence last week.

Thomas, who was information commissioner from December 2002 to June 2009, said he had been shown "cardboard boxes full of material" by ICO investigators in 2003, seized in a raid on the home of private investigator Steve Whittamore days earlier.

He insisted he had kept an open mind over whether to charge editors and journalists after viewing the evidence it contained and denied adopting a policy that ruled ruling this option out.

"If there was a policy it was not one that I had a hand in, one that I knew about, that I made or that I was told about," he said. "I was interested in what they had found, but our targets were the investigators because they were the middle men."

Thomas conceded that the legal advice the ICO received from counsel Bernard Thorogood in 2003 had made it clear that there was sufficient evidence for prosecutions of journalists.

The information seized from Whittamore showed that 305 journalists had been involved in over 13,000 transactions with the private investigator, of which 5,025 had breached the terms of the Data Protection Act.

"I was told some time in October or November [2003] that it was going to be too expensive or too difficult to pursue the journalists. That's when I went off to the PCC," said Thomas. "It was not an ideological, strategic policy – it was a practicality."

Whittamore and three co-defendants were eventually convicted and given two-year conditional discharges at Blackfriars crown court in 2005.

Thorogood advised Thomas in January 2005 that police, who had been investigating Whittamore and his associates for alleged corruption over the trade in criminal records, vehicle registration and telephone company data, had found it difficult to get information out of reporters. "The journalists were interviewed and were found to be tricky, well armed and well briefed – effectively a 'barrel of monkeys'," minutes from the meeting noted.

Thomas said he was advised by counsel not to take action against journalists at that point, which he reluctantly accepted. Thomas said he had raised the use of private investigators with Sir Christopher Meyer, then chairman of the Press Complaints Commission, in writing and in person two years before that. He also met PCC representatives and members of the editor's code of practice committee, including Daily Mail editor-in-chief Paul Dacre and Les Hinton, then chairman of the Sun's owner, News International.

Thomas said he was "disappointed" with the PCC's failure to act despite a series of meetings. "In hindsight I would have been more aggressive and assertive with the PCC," he said.